1. Field of the Invention
This invention relates to microstrip devices, such as antennas and filters, and in particular to a method for tuning such devices by applying a plastic dielectric substance on the surface of the device.
2. Related Art
Microwave devices designed to have coupled line structures, particularly antennas and filters, depend primarily on odd mode interline coupling capacitance for setting of the edge frequency of the pass band. Below this frequency the device rejects unwanted signals. These devices are typically implemented on planar media, such as fiber-glass epoxy circuit boards.
The dielectric constant of fiber-glass epoxy circuit boards is not well controlled. Variation in the dielectric constant of the circuit board changes the odd mode interline capacitance. With filters, this causes shifting of the pass band edge. There is thus a need to reduce the variation in odd-mode capacitance, or at least to compensate for shifts in the odd-mode capacitance from a design or target value.
Various ways are known for tuning a microstrip device after it is constructed and found to have odd-mode capacitance that varies from the design value. For instance, an electrically conductive plate may be suspended over the device. The closer the plate is to the device, the greater the coupling. This, however, is an expensive solution.
Techniques have also been used that involve positioning a dielectric layer on top of the microstrip device. Jecko et al., in U.S. Pat. No. 4,638,271, discloses placing a dielectric plate having one of a variety of shapes on the microstrip conductors of a filter. Once the plate is attached, adjustments are made by making cuts with a scalpel until a desired value is obtained, or by adding or removing strips of the dielectric material. Additionally, adjustment may be made by machining away the thickness of the plate or by adding additional layers having the same or different dielectric constant. This technique, though ultimately effective, requires a lot of labor, inventory of dielectric materials, and special apparatus to effect it. It is therefore also expensive and time-consuming to perform.
An alternative approach is to silk-screen one or more layers of a dielectric paint or ink on the microstrip device. This approach, taught by Andrews in U.S. Pat. No. 4,706,050, requires specialized silk-screening equipment, which must be maintained, as well as following a multistep process when more than one layer is required. Further, each layer alters the frequency of tuning of the device by a specific amount, making it difficult to precisely tune the device.
There thus remains a need for a technique for tuning a microstrip device that is simple to perform yet effective for precisely tuning the device.